A common current sensor type relies on the principle that an alternating electric current carried within a conductor induces a magnetic flux. The magnetic flux may then be used to induce a current in a conductive coil. The induced current is proportional to the current flowing in the conductor. A magnetically permeable core used in such sensors enhances the properties of the sensor by focusing the magnetic flux relative to the conductive coil. Measurement of the electric current flowing in the coil therefore provides a measure of the electric current flowing in the conductor. The coil may further form a primary coil of a current transformer, and therefore provide a power supply to electronics associated with the sensor making the device a standalone, self-powered device. This relatively simple principle, however, finds difficulty in implementation.
One difficulty is that to take full advantage of the magnetic flux it is best that the permeable path encircle the conductor. Should the permeable path not encircle the conductor, the sensor is more susceptible to interference from other magnetic sources. However, to accomplish this, the conductor has to be made to pass through the permeable core, typically formed as a toroid. For long, high-voltage transmission lines as are commonly used in the electrical power distribution field, this means undesirably breaking or severing of the conductor so that the core may be positioned at the desired location. Feeding the conductor through the core during installation of the line is impractical as it may require feeding literally miles of conductor through the coil, and this option is not available where the sensor is to be installed on an existing installation.
Positioning a permeable core adjacent to the conductor, but not through the core still results in the development of a magnetic flux about the core and a current in a coil encircling the core. One proposal in this regard is to provide a coil in a line post/insulator adjacent to a conductor support portion of the insulator. As noted above, this design is less effective than if the core were made to encircle the conductor and it results in the sensor being substantially more susceptible to noise and interference.